1. Field of the Invention
The present invention relates to a method and a device for aligning a video image with an edge of a display screen.
2. Discussion of the Related Art
Generally, a video image is displayed on a display screen of a display terminal by exciting phosphors arranged on the screen by means of one or several electron beams, emitted by electron guns. In the case of a color screen, a cathode-ray tube with three electron beams, each exciting a type of phosphor respectively emitting a red, green, or blue light, is generally used. The electron beams are modulated in intensity by modulation signals representative of the image to be displayed on the screen.
Generally, the electron beams are focused at one point of the screen and are deviated together to scan screen lines. The electron beams scan the screen lines from the right to the left of the screen, returning to the left of the screen after the scanning of each line. The screen scanning is performed from the upper horizontal edge to the lower horizontal edge.
The electron beam deviations are obtained by two deflection coils, one horizontal deflection coil which controls the scanning of each screen line, and one vertical deflection coil which controls the deviations of the electron beams in the vertical direction.
FIG. 1 shows a conventional example of a timing diagram of current I supplying the horizontal deflection coil. For clarity, the timing diagram is not shown to scale. Curve 10, representative of current I, comprises linear ascending ramps 11 linked by descending portions 12. Each ascending ramp 11 corresponds to the scanning, by the electron beams, of a line from the left to the right of the screen. Each descending portion 12 corresponds to the return of the electron beams to the left of the screen.
FIG. 2 shows timing diagrams of signals implemented to form ramps 11 of current I and to display an image on the screen.
From the image signal received by the display terminal, is extracted a square signal HSYNC, represented by curve 15, from which are generated the different clock signals synchronizing the elements of the display terminal. Blocks of video signals VIDEO, shown as blocks 16A, 16B of curve 16, each block 16A, 16B comprising the video data necessary for the display of an image line, are also extracted from the image. Each video block 16A, 16B is transmitted to a modulation system to generate the signals for modulating the intensity of the electron beams.
A rising edge of signal HSYNC indicates the beginning of the display of a line. Duration ΔTVIDEO separating one rising edge of signal HSYNC from the beginning of the transmission of a video block 16A, 16B is constant.
The stopping of an ascending ramp 11 is controlled by the rising edge of a signal HOUT, represented by curve 17. Signal HOUT is provided by a device for controlling the display terminal based on signal HSYNC. Signal HOUT is, for example, a square pulse signal, each rising edge of which, separated by a duration ΔTHOUT from the preceding rising edge of signal HSYNC, corresponds to the end of an ascending ramp.
The display frequency of the lines on a screen is generally high, for example, on the order of some thirty kilohertz, whereby the implementation of ascending ramps 11 of current I supplying the horizontal deflection coil must be performed directly at the level of the power stage of the horizontal deflection coil by discrete components. The control device transmits signal HOUT to the power stage of the horizontal deflection coil to control the end of each ascending ramp 11. The power stage transmits in return to the control device a signal HFLY, represented by curve 18, each pulse of which indicates a descending part 12 of the supply current of the horizontal deflection coil, that is, the return of the electron beams to the left of the screen.
Response times tS of the power stage components may be variable. There may thus appear a variable duration between the reception of signal HOUT by the power stage and the end of the associated ascending ramp 11. Now, it is necessary to ensure a constant duration ΔTVID between signal HFLY and the beginning of the transmission of the next video data block 16A, 16B so that the pixel display on a screen line starts at a substantially constant distance from the screen edge.
FIG. 3 shows in the form of a block diagram a conventional control device 20 implementing a regulation of duration ΔTVID between signal HFLY and the beginning of the transmission of the next video data block 16A, 16B.
Control device 20 receives as inputs signal HSYNC and signal HFLY. Signal HSYNC is transmitted to a counter 22 which, after a variable delay ΔTHOUT, transmits signal HSYNC to a calculation unit (SYNT.) 24 which provides signal HOUT. Control device 20 also comprises a comparator 26, which determines duration ΔTHFLY separating the reception of a rising edge of signal HSYNC and of the next signal HFLY, compares the measured duration with a reference duration ΔTREF, and accordingly provides an error signal to a correction unit 28. Correction unit 28 transmits to counter 22 a new duration ΔTHOUT to be used to correct the position of the next pulse HOUT to obtain a better position of the next signal HFLY.
The correction performed on duration ΔTHOUT will affect the next signal HOUT transmitted after signal HSYNC. Thereby, the ascending ramp 11 which immediately follows the signal HFLY from which the correction has been determined, is not corrected, since it originates from an already-transmitted signal HOUT. The video block immediately following signal HSYNC may thus not be correctly synchronized with ascending ramp 11. The line of the video image associated with the video block may be displayed in a shifted manner with respect to the screen edge while the next line of the video image will be properly displayed.